Allowed U.S. Pat. No. 5,539,730 entitled "Hybrid FDMA/TDMA/CDMA Access Methods" discloses the advantages of asymmetrically channel spaced TDMA systems using short bursts of high data rate, wideband transmission from a network to a battery operated receiving terminal while using longer bursts of narrowband transmission for transmission from the battery operated terminal to the network. U.S. Pat. No. 5,539,730 is incorporated herein by reference.
In analog cellular telephones such as those designed for the U.S. AMPS system for example, conventional single-loop digital frequency synthesizers can be employed. In such phones the channel spacing is 30 KHz and there are no requirements for extremely fast channel changing. In the pan-European digital cellular system GSM, frequency hopping is employed to mitigate the effects of slow fading. Fast frequency switching synthesizers are then needed, but as the channel steps of 200 KHz are relatively coarse, the switching speed can just be achieved by conventional techniques. Both of the above systems employ the same channel spacing for each direction of communication, i.e. base-to-mobile communication and mobile-to-base communication.
In the U.S. digital cellular system IS54, also known as Digital AMPS or D-AMPS, the channel spacing is the same as its analog predecessor AMPS at 30 KHz, but a requirement for fast frequency changing arises to permit mobile phones to scan the frequencies of surrounding base stations during short idle periods between transmit and receive bursts. U.S. Pat. Nos. 5,095,288 entitled "Variable Bandwidth Synthesizer" and 5,180,933 entitled "Fractional-N Synthesizer" describe techniques useful in obtaining fast frequency switching speeds and are both incorporated herein by reference. A synthesizer integrated circuit incorporating the teachings of the above patents is marketed by Philips as part no. UM8025.
In "Phaselock Techniques" (Floyd M. Gardener, Wiley 1979, ISBB 0-471-024294-3) Gardener illustrates a dual-loop synthesizer called "vernier loop" which permits the generation of small frequency steps while keeping the phase comparison frequency of both loops high. This is in the prior art a known alternative technique for achieving small frequency steps, fast switching and low phase noise.
In the parent disclosure, a novel synthesizer is described employing two synthesizers each conforming to the teachings of the above-incorporated patents and arranged in a vernier-loop configuration so as to achieve the switching speed advantages of both the fractional-N technique and the Vernier-loop technique. Such a synthesizer was disclosed to provide both narrow frequency spacings when communicating through a satellite relay with a ground network or alternatively coarser channel spacings when communicating directly with a land-based station.
In this disclosure, an advantageous frequency synthesis arrangement using the teachings of the parent application is described for providing narrow frequency spacings for transmission alternately with providing coarse frequency spacings for reception when communicating with a land network with or without the use of an orbiting relay satellite, the communication method being an asymmetrical TDMA method according to the above incorporated documents.